Base-station antennas for 3 sector systems shall typically have 65 degrees horizontal beamwidth for good coverage and small interference. This beamwidth can also be desirable in other configurations of base station antennas. The generic beamwidth of a single antenna element on a large ground plane is typically much larger, 80-100 degrees. FIG. 1 presents a perspective view of one of the antenna elements 101 in a single polarized base station antenna with one column over a reflector or ground plane 102. The base station antenna is located in a coordinate system with an x-axis 104, a y-axis 105 and a z-axis 106. The ground plane is mainly extending in a y/z-plane and the antenna element is extending in an antenna element plane being substantially a parallel y/z-plane with a different x-coordinate. Several antenna elements are located over the ground plane along a column axis 107 being parallel to the z-axis. The antenna elements are elongated dipoles with a longitudinal extension of the dipoles having an antenna element angle α in clockwise direction to the column axis 107 of 45 degrees. The beamwidth can be reduced to 65 degrees for the single polarized antenna of FIG. 1 and also for a standard dual-polarized base station antenna with one column by proper design of the reflector or ground plane width 103. The total width of such design is typically 0.9-1λ when the beamwidth is 65 degrees. λ denotes the average wavelength in the operating frequency band of the antenna. The elongated reflector or ground plane is extending in the direction of the z-axis as indicated by the dash dotted lines 108.
The difference between a single polarized antenna and a dual polarized antenna is the antenna element. In a single polarized antenna as shown in FIG. 1 the antenna element can typically be a single dipole element. In a dual polarized antenna the antenna element typically comprises two crossed dipoles with a 90 degree angle between the longitudinal extensions of the two dipoles.
Horizontal co-polarized farfield radiation patterns 203, henceforth in description and claims called the radiation pattern, in the frequency range from 1700 to 2200 MHz are shown in the diagram of FIG. 2a for the antenna arrangement of FIG. 1 with a reflector or ground plane width of 0.9λ and with one antenna element. In FIG. 2a amplitude is shown in dB on the vertical axis 201 and direction in degrees in relation to the x-axis on the horizontal axis 202. The radiation pattern has approximately rotational symmetry around the x-axis with the maximum field pointing in the positive direction of the x-axis, corresponding to zero degrees in FIG. 2a. The direction of 90 degrees thus corresponds to the radiation in the antenna element plane. Beamwidth 204 is shown in FIG. 2b for the antenna arrangement of FIG. 1 with one antenna element and with a 0.9λ wide reflector or ground plane 102 with beamwidth in degrees on the vertical axis 205 and frequency in MHz on the horizontal axis 206. The radiation pattern shows a beamwidth in the 65-70 degree interval. The diagrams of FIG. 2 are valid either for a single polarized antenna or for each of the polarizations from a dual polarized antenna.
For smaller reflector or ground plane widths it is not possible to achieve a 65 degree beamwidth. FIGS. 3a and 3b presents typical results for a 0.7λ wide reflector or ground plane and with the antenna arrangement of FIG. 1 with one antenna element. The radiation patterns 303 in the frequency range from 1700 to 2200 MHz are shown in the diagram in FIG. 3a having amplitude in dB on the vertical axis 301 and direction in degrees in relation to the x-axis on the horizontal axis 302 in the same way as described for FIG. 2a. Beamwidth 304 is shown in FIG. 3b for a 0.7λ wide reflector or ground plane 102 with beamwidth in degrees on the vertical axis 305 and frequency in MHz on the horizontal axis 306. The radiation pattern shows a beamwidth in the 75-80 degree interval.
In typical base station antennas several columns in parallel are normally used in order to improve possibilities for digital beam forming and use of the Multiple Input Multiple Output (MIMO) principle.
When using digital beam forming, which is a well know technology for the skilled person, the beam of the antenna is directed or scanned in different directions by e.g. feeding the transmitted signal to the antenna elements with different time delays.
FIG. 4 shows a perspective view of a one antenna element section of a 4-column antenna with a first 401, a second 402, a third 403 and a fourth 404 column. The antenna is located in a coordinate system with an x-axis 405, a y-axis 406 and a z-axis 407. Each antenna element 408 in a column is extending in a plane substantially in parallel with a separate column ground plane 409 for each column. The ground planes are mainly extending in a y/z-plane. A column separation 410 is defined as the distance between midpoints 413 of neighboring column ground planes 409. A ground plane separation 411 is defined as the separation between neighboring ground planes and a column width 412 is defined as the width of a column ground plane. Each column ground plane is elongated and has a longitudinal extension in the direction of the z-axis 407. The longitudinal extensions of the different ground planes are typically substantially in parallel. The ground plane separation 411 is normally very short compared to the column width 412, which means that the column separation 410 normally is about the same as the column width 412.
Dash dotted lines 414 in the forth column 404 schematically indicates how the elongated column ground planes are extending in the direction of the z-axis. Antenna elements are extending above the column ground planes as explained in association with FIG. 1.
The column separation of multi-column antennas is a critical parameter which is subject to several conflicting requirements:                Antenna size                    Smaller column separation results in a smaller antenna with less visual impact and wind load.                        Grating lobes during beamscan                    0.5λ column separation enables large scan angles without any grating lobes            1.0λ column separation gives grating lobes for moderate scan angles                        Correlation                    Larger column separation decrease correlation which improves MIMO qualities.                        Antenna beamwidth                    From a network perspective (assuming e.g. 3-sector sites) 65 degree beamwidth is preferred.                        
Grating lobes are side lobes dependent on the antenna element spacing in array antennas. The amplitude of the grating lobe can be comparable to the main lobe when the beam is scanned for antenna element spacings larger than 0.5λ. In this case the antenna element corresponds to one column of antenna elements and the spacing between antenna elements corresponds to the column separation.
To meet the requirement to have a narrower beam, around 65 degrees, for each column in a multi-column antenna and simultaneously a compact antenna, the same considerations apply as described for a single column antenna in association with FIGS. 1-3 except that the column width parameter now is replaced by the column separation parameter. This has the consequence that, by using existing technology, it is not possible to achieve a 65 degree beamwidth for column separations smaller than 0.9λ. For example, an antenna with a 65 degree beam having a 0.7λ column separation is not possible.
There is thus a need for a multi-column antenna with a narrower beamwidth and simultaneously having a narrower column separation than the prior art solutions of today.